Fiber optic sensor systems generally rely on varying the intensity of a light beam or upon measurements based on the wavelength of light. The first approach is analog in nature and, therefore, limited for high performance applications. The second approach is capable of greater performance but still has limitations.
An example of the limitations of the second approach is based upon light interference. The wavelength of a typical coherent light source is 0.83 micrometers or 33 micro-inches. An optical resonant cavity designed for this wavelength will display interference maximums and minimums at one half wavelength intervals, or 16.5 micro-inches. If strictly digital performance is desired, then, these interferences must be counted to measure a displacement. A high performance sensor, however, can easily require one part in one million resolution so that the required displacement for this type of measurement would be 16.5 microinches.times.one million=16.5 inches. Since this is generally not practical for parameters such as high pressure, a compromise must be msde. For example, a displacement of 50 interferences would be 0.0008 inches, which would give a resolution of 2%, based on digital techniques. Any additional required resolution for this maximum displacement would then have to be obtained from analog interpolation of phase information to further resolve the displacement between interferences.
Other disadvantages of these wavelength of light approaches are the requirement for single mode fibers and potentially sophisticated, expensive readout interface equipment. Single mode fibers are required to preserve the coherency of the light source in the optical system. These fibers have to have an extremely small diameter core (on the order of 0.0004 inches) and therefore are difficult to splice, connect and handle. The disadvantages of complex, expensive readout equipment speak for themselves.